I buy locomotives that run with MTH's DCS, so the operating system makes me brand loyal. However, every once in a while Lionel may produce a locomotive that MTH does not, and I must have it. The only case so far is the TMCC NYC S3 Electric, the icon of model railroading. In the future I plan to convert the S3 to PS32 so I can enjoy running it.
MTH Rail King locomotives have been outstanding on my railroad, so I am partial to Rail King.
As for trolley cars, passenger and freight rolling stock, I am not brand loyal.